Computing devices, such as personal computers, tablet computers, mobile phones, or other devices, often request content from other computing devices over a network. In a common application, the computing device operated by a requesting user is referred to as the client, and the computing device or system operated by the content provider that responds to the request is the server. A client and server may communicate over an intranet, the Internet, or any other communication network.
Clients receive and process content (e.g., web pages) from content servers. In many cases, the requested content changes over a period of time, or from request to request. Changes at the content server may not be reflected in copies of content previously received by clients. Clients may receive the changes by again requesting and receiving the content from the content server.